SMS Mode

The Sega Genesis is natively backwards compatible with the Sega Master System. In fact, most of the hardware contained in the Master System is also found in a Genesis: Z80 processor, PSG, etc…

To enter Master System compatibility mode, it seems that you must, at the very least, do the following (refer to the Pinouts section for Genesis pin locations):

Genesis Pin B30 (#M3) – connect to GND

Genesis Pin B9 (A21) – connect to VCC

Genesis Pin B10(A22) – connect to GND

With the above connections, the cartridge port loses the #RESET signal on B27 – it now constantly outputs GND instead of a nice active-low reset pulse when the reset button is pressed. Therefore, you cannot rely on B27 for reset when in Master System compatibility mode. This is the source of much frustration for me! The cartridge can still send a reset signal to the console via pin B2 in Master System mode.

Pin Reconfiguration

Other Genesis pins change functions when entering Master System compatibility mode. There is very little information available on these, the information listed below was found on SMSPower.org and through experimentation. The list is not exhaustive and I will keep adding to it as I discover more…